In the process of making paper, an aqueous suspension of fibers is transformed into a paper web as it is processed through different sections of a paper making machine. One section of the paper making machine is the dryer section wherein a wet paper web is passes about and held in intimate heat transfer contact with upper and lower arrays of heated cylinders in order to remove the water from the paper web and dry it completely. The dryer section normally includes an upper and lower array of heated cylinders arranged and spaced in staggered, parallel rows which have a solid imperforate surface for contacting the paper web. The paper web passes between the arrays of dryer cylinders in a generally serpentine manner to ensure that both sides of the paper web contact the cylinders and dry evenly. As the paper web passes over the dryer cylinders, it is held in intimate heat transfer contact by a belt commonly known as a dryer belt or dryer fabric which has been made endless by techniques that are well known in the field of papermaking felts and clothing.
The heated cylinders of the dryer section are typically heated by steam introduced into the interior of the cylinders, by infrared radiation, or by other suitable means. However, in a lot of cases the temperature of the heated cylinder varies from one end to the other by a considerable amount. This creates the possibility that some portions of the paper web will be subjected to greater drying action than other portions of the paper web. The variance in the cylinder temperature and the drying action creates a non-uniform moisture profile in a paper web across the width of the web. Furthermore, it is known that the lateral edges of the paper web are more readily ventilated since they lie closer to the surrounding atmosphere and thus they tend to dry more quickly and more completely. This phenomenon is commonly known as rimming.
To eliminate the higher moisture in the center of the paper web, numerous methods and mechanical arrangements have been proposed. For example, mechanical corrections to the machinery have been implemented to try to avoid the uneven heating of the cylinders and non-uniform exposure of the paper web to the atmosphere. Proceeding in a different direction, the paper drying fabric itself has been designed to alleviate this problem by building in a desired permeability profile into the dryer fabric so that the resulting drying of the paper web is uniform across its width from one end of the heating cylinder to the other. For example, in U.S. Pat. No. 3,867,766 a woven dryer fabric is disclosed in which the number of warp elements at the lateral edges is varied to produce a desired permeability profile across the edges and the middle of the fabric. In U.S. Pat. No. 4,460,023 it is proposed to insert an additional pick of the weft yarn across the width of the fabric at the lateral edges of the fabric to reduce the permeability at the edges. The remainder of the pick across the width of the fabric is severed so that the pick of the weft yarn only exists at the lateral edges. While the above constructions are satisfactory for woven constructions, these fabrics and methods do not lend themselves readily to producing a profile permeability in non-woven dryer fabrics.
Helical dryer fabrics which include a number of helices joined together by intermeshing the windings of the helices and extending a pintle through the channel formed by the intermeshed windings have become increasingly in use as dryer fabrics. U.S. Pat. No. 4,381,612 discloses such a dryer fabric wherein monofilament stuffer elements are inserted through the winding spaces of the helices across the fabric width in order to cut down on the overall permeability of the fabric. Owing to the considerable openness of the helical mesh which exists in the fabric, the blocking of the fabric is necessary in many applications to avoid unnecessary pumping of air through the fabric and resultant paper flutter against the fabric. However, even in the use of a helical dryer fabric, the problem still exists of uneven temperatures across the width of the heating cylinders which results in uneven drying of the paper.
In U.S. Pat. No. 4,192,080 a method of improving the drying characteristics of a cylinder drying section of a paper making machine is disclosed which includes coating the rim areas of the outer jacket surface of at least some of the drying cylinders with a heat insulating foil.
Thus, while many mechanical expedients and fabric constructions have been proposed for woven fabrics, the provision of a non-woven helical dryer fabric having a desired permeability profile is a problem to which much attention need be given.
Accordingly, an important object of the present invention is to provide a non-woven, helical dryer belt having a desired permeability profile across the width of the fabric to produce uniform drying of a paper web in a paper making machine.
Still another important object of the present invention is to provide a dryer belt and method for providing a profile permeability characteristic in a paper making fabric so that a uniform moisture content across the width of a paper web being dried is obtained.